Performance gain flattened EDFA

ABSTRACT

The invention is directed to a multistage optical amplifier having a gain stage between a GFF and an optical attenuator. More specifically, in accordance with an illustrative embodiment of the present invention, an optical amplifier system comprises multiple gain stages arranged serially for providing gain to an optical signal propagating therein. The system also includes a gain flattening filter for reducing a variation in gain of the optical amplifier system in a particular wavelength band and an optical attenuation element. One of the gain stages is located between the gain flattening filter and the optical attenuation element to reduce an overall noise factor of the optical amplifier system. Illustratively, each gain stage is a pumped segment of erbium doped optical fiber. The optical attenuation element may precede the gain flattening filter in a direction of propagation of an optical signal in the amplifier system. Alternatively, the gain flattening filter precedes the attenuation element in the direction of propagation of the optical signal. In an alternative embodiment of the invention, a gain flattened optical amplifier system comprises a plurality of optical gain stages and a plurality of gain flattening filter stages arranged serially in an alternating manner.

This application claims priority to U.S. Provisional Application No.60/080,127 filed Mar. 31, 1998 which is herein incorporated by referencein its entirety.

FIELD OF THE INVENTION

The present invention relates to a technique for improving theperformance of a multistage Erbium Doped Fiber Amplifier (EDFA) whichcontains a gain flattening filter (GFF) and possibly other lossycomponents including a variable optical attenuator (VOA).

BACKGROUND OF THE INVENTION

It is important for an optical amplifier used in awavelength-multiplexed communication system to have a uniform or flatgain spectrum. An EDFA can produce gain over a spectral width of morethan 30 nm, but even under optimum pumping conditions the gain spectrummay not be uniform. A gain flattening filter (GFF) is known to be usefulin optical amplifiers to reduce the variation in the gain over some bandof wavelengths (see e.g., M. Tachibana, R. I. Laming, P. R. Morkel, andD. N. Payne, “Erbium-doped fiber amplification with flattened gainspectrum,” IEEE Photonics Technology Letters, vol. 3, pp. 118-120,1991). Static GFFs, however, can only provide optimum gain-flatness at asingle gain value (i.e., gain at any particular wavelength). If the gainof an EDFA is changed by changing the inversion (e.g., by changing thepumping power or signal power), the gain changes in a spectrallydependent manner pumping power or signal power (see, e.g., C. R. Gilesand D. J. D. Giovanni, “Spectral dependence of gain and noise inErbium-doped fiber amplifiers,” IEEE Photonics Technology Letters, vol.2, pp. 797-800, 1990 and J. Nilsson, Y. W. Lee, and W. H. Choe, “Erbiumdoped fiber amplifier with dynamic gain flatness for WDM,” ElectronicLetters vol. 31, pp. 1578-1579, 1995). As a result, if a conventionalEDFA is used in an application where its gain needs to be different fromthe design gain of the amplifier, its gain spectrum will show excessnormalized gain ripple ((maximum gain—minimum gain)/minimum gain) ascalculated in the wavelength band of interest. An example of how thiscan be a problem is provided by an optically amplified fibertransmission system where one needs to support fiber spans shorter thanthose for which the amplifier is designed. It is typically impracticalto have separate amplifiers custom designed for each fiber span.Therefore, one is either forced to have an amplifier with a distortedgain spectrum or to add enough loss to the system so that the designgain is actually needed from the amplifier. An optical attenuatorintentionally added to a system for gain-flattening purposes will tendto increase the amount of noise that is added to the signal and requireadditional pump power relative to a similar cascade of amplifiers thathave been redesigned to provide flat gain at the actual gain levelneeded. This performance loss can be reduced by placing the addedoptical attenuation between the gain stages of a multistage opticalamplifier (see, e.g., Y. Sugaya, S. Kinoshita, and T. Chikama, “Novelconfiguration for low-noise and wide-dynamic-range Er-doped fiberamplifier for WDM systems,” in Optical Amplifiers and theirApplications, 1995 OSA Technical Digest Series, Vol. (Optical Society ofAmerica, Washington, DC) 158-161 and N. E. Jolley, F. Davis, and J. Mun,“Out-of-band electronic gain clamping for a variable gain and outputpower EDFA with low dynamic gain tilt,” in Conference on Optical FiberCommunication, 1997 OSA Technical Digest series, Vol. 6, (OpticalSociety of America, Washington, D.C.) 134-135).

It is an object of the invention to utilize this approach of placingattenuation between gain stages in an amplifier that contains a GFF,while maintaining good optical performance. This is especially importantfor wideband EDFA's where the GFF needed may attenuate only parts of thegain spectrum. This approach is also applicable when a high attenuationGFF is used alone or with other attenuating optical elements.

It is well known that the impact that a GFF has on the performance of anoptical amplifier can be reduced by properly inserting the GFF betweentwo gain stages (see e.g., M. Tachibana, R. I. Laming, P. R. Morkel, andD. N. Payne, “Erbium-doped fiber amplification with flattened gainspectrum,” IEEE Photonics Technology Letters, vol. 3, pp. 118-120,1991). For a conventional line amplifier the negative impact of the GFFcan be significantly reduced with proper filter placement. However, asamplifiers move toward wider bandwidths, the impact of the GFF becomesmore significant for a number of reasons. Wider bandwidths tend torequire GFFs with larger peak attenuations. As the peak attenuation of afilter increases, its negative impact on amplifier noise/output powerperformance will generally increase also. Wide bandwidth amplifiers alsofrequently make use of the short wavelength portion of the erbium gainspectrum (or “blueband”) which roughly extends from 1525-1540 nm. It istypically harder to achieve optimum noise performance in this part ofthe spectrum since the intrinsic noise performance of the amplifyingfiber is more sensitive to the local inversion. As illustrated below,these effects can compound each other in multistage amplifiers where thefinal power stages may have very low inversions.

More particularly, FIG. 1 schematically illustrates a three stage EDFA10. An optical signal to be amplified enters the EDFA 10 at input port11 and the amplified optical signal exits the EDFA at output port 12.The EDFA 10 includes three gain stages whose power gains are designatedas G₁, G₂, G₃. Each of the gain stages G₁, G₂, G₃ comprises a pumpedsegment of erbium doped optical fiber. The erbium dopant providesoptical gain for optical radiation propagating in the optical fibersegment. Alternatively, it may be possible for other elements besideserbium, such as the rare earth elements, to provide the appropriategain.

In FIG. 1, T_(i) is the net (linear) transmittance up to the ith gainstage. Therefore, T_(i) is the product of the linear transmittancefactors of lossy components and the gain factor for amplifyingcomponents. Thus, T_(i) may be viewed as the power transmissioncoefficient (accounting for insertion loss) for all components with theindicated position relative to the gain stages, G_(i), i=1, 2, 3. Thequantities T_(i) and G_(i) may be a function of wavelength.

F_(total)=F₁/T_(in−1)/+F₂/T_(in−2)+F₃/T_(in−3)  (1)

where F₁ is the noise factor (linear units) of the first gain stage andT_(in−1) is the net linear power transmittance of all the opticalcomponents from the amplifier input to the beginning of the first gainstage. The symbols associated with the noise contributions of the otherstages are analogously defined. For a high gain amplifier (G>˜20 dB) theminimum possible value of F₁ is 2. One typically designs opticalamplifiers such that the first term in Eqn. 1 dominates the total noisefactor while operating with F₁ as close to the quantum mechanical limitas possible (high population inversion). Because of the low inversionstypically used in subsequent gain stages (high pump to signal powerconversion efficiency is typically easiest for attaining a lowerinversion), the only way to make the impact of these stages on theoverall amplifier noise factor small is to make

T_(in−1)=T₁G₁+. . . +T_(i−1)G_(i−1)  (2)

i.e., the net power transmission factor from the amplifier input to thebeginning of the ith stage, as high as possible. This can typically beaccomplished by using a high gain (˜20 dB) in the first stage. Highergains are difficult to attain with a single stage due to the buildup ofamplified spontaneous emission. However, GFF peak attenuations for wideband amplifiers frequently approach or exceed 10 dB while a 10 dB gaindynamic range would require a (variable) optical attenuator with a peakattenuation >10 dB (10 working range+minimum loss). Furthermore, thenoise factor of the low inversion stages can be in a range from near 10to larger than 10. If components with insertion losses such as these areimmediately cascaded within the amplifier, their aggregate attenuationwould result in low values for T_(in−i) and the overall noise factor ofthe amplifier would be affected.

It is a further object of the invention to overcome this problem so asto improve the performance of a multistage EDFA.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the invention, theabove-described problem of the prior art is reduced and an improvedmultistage optical amplifier is achieved. This is accomplished accordingto the invention by providing an additional gain stage between a GFF andan optical attenuator in a multistage optical amplifier such as amultistage EDFA.

More specifically, in accordance with an illustrative embodiment of thepresent invention, an optical amplifier system comprises multiple gainstages arranged serially for providing gain to an optical signalpropagating therein. The system also includes a gain flattening filterfor reducing a variation in gain of the optical amplifier system in aparticular wavelength band and an optical attenuation element.Illustratively, the optical attenuation element includes one or more of:a variable optical attenuator, a switch, an isolator, an addmultiplexer, and a drop multiplexer. One of the gain stages is locatedbetween the gain flattening filter and the optical attenuation elementto reduce an overall noise factor of the optical amplifier system.Illustratively, each gain stage is a segment of erbium doped opticalwaveguide which is pumped. The optical attenuation element may precedethe gain flattening filter in a direction of propagation of an opticalsignal in the amplifier system. Alternatively, the gain flatteningfilter precedes the attenuation element in the direction of propagationof the optical signal.

In general, one can put more gain in the early stages and improve thenoise figure for any design. This however, will typically reduce thepump power to signal power conversion efficiency. Thus, noiseperformance and pump power requirements are trade-offs in many designsof interest. In many cases, what can be achieved according to theinvention is an improvement of one (noise figure or pump powerrequirement) at a fixed value of the other.

In an alternative embodiment of the invention, a gain flattened opticalamplifier system comprises a plurality of optical gain stages and aplurality of gain flattening filter stages arranged serially in analternating manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a multistage EDFA.

FIG. 2A schematically illustrates a multistage EDFA with a variableoptical attenuator (VOA) separated from a GFF by a gain stage and inwhich the GFF precedes the VOA in the direction of propagation of theoptical signal, in accordance with one embodiment of the invention.

FIG. 2B schematically illustrates a multistage EDFA with a VOA separatedfrom a GFF and in which the VOA precedes the GFF in the direction ofpropagation of the optical signal, in accordance with a secondembodiment of the invention.

FIG. 3 illustrates a multistage EDFA comprising a first GFF preceding again element and a second GFF following the gain element.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2A illustrates an optical amplifier system in accordance with anillustrative embodiment of the present invention. The optical amplifiersystem 20 includes an input port 22 through which optical radiation tobe amplified enters the system and an output port 24 through whichamplified radiation leaves the amplifier system. Illustratively, theamplified optical radiation has a wavelength in the range of the erbiumgain band, e.g., 1530-1560, though it could be much wider than this.

The amplifier system 20 comprises three gain stages whose power gainsare designated G₁, G₂, G₃. Illustratively, each gain stage comprises anErbium doped optical fiber amplifier. Such an amplifier is disclosed inU.S. Pat. No. 5,710,659. The stage G₁ typically has relatively high gain(15-20 dB). The other stages would likely have lower gains; stage G₂roughly 10-15 dB, stage G_(3,) 5-10 dB. These values depend highly onthe amount of attenuation contained within components inside theamplifier (such as GFF and VOA, but also other components such asdispersion compensators, optical add/drops) and the choice of the gainband (wide gain bands will require GFFs with higher peak attenuation.)Ti represents the power transmission coefficients of any opticalcomponents between the input port 22 and the first gain stage G₁. (Inmany cases, there may be no such components.) A GFF 26 is locatedbetween gain stages G₁ and G₂. The GFF has a peak attenuation of 10 dB,for example. GFF peak attenuation depends on width of bands, and howmuch gain is provided by the erbium fiber which is the external gainprovided by the amplifier minus the total attenuation of all components.A peak attenuation of 10 dB can be had for the range 1530 nm to 1560 nmfor total erbium gain of 40 dB or more, for the types of erbium dopedfiber typically used. An attenuating element 28 is located between gainstages G₂ and G₃. The attenuating element 28 may include an attenuator,a VOA, a switch, or an add/drop element, or another element withattenuation or a combination of such elements. The attenuating element28 has an attenuation which is greater than 10 dB, for example.

The purpose of separating GFF 26 and attenuating element 28 by a gainstage (i.e., gain stage G₂) is to achieve a high value for T_(in−1) andalso a low noise factor F_(total). Since T_(i) is in the denominator ofEqn. (1), keeping its value high keeps F_(total) low. If the GFF andattenuating element were directly cascaded to one another within theamplifier system, their aggregate attenuation would result in either lowvalues in T_(in−1) and results in adversely affected overall noisefactor F_(total) of the amplifier or alternatively results in a poorpower conversion efficiency. The present invention overcomes thisproblem by separating the GFF and attenuating element by a gain stage.

FIG. 2B schematically illustrates an alternative embodiment of theinvention. The optical amplifier system 20 of FIG. 2B is similar to theamplifier system of FIG. 2A. The difference is that attenuating element28 is located between gain stages G₁ and G₂ and GFF 26 is locatedbetween gain stages G₂ and G₃. The amplifier system 20 also achievesimproved noise performance by avoiding the direct cascading of theattenuating element 28 and GFF 26.

The choice of which configuration is best (i.e., FIG. 2A or FIG. 2B)will depend on the spectral band of interest as well as other details ofthe amplifier design. Therefore, it will need to be determined on acase-by-case basis. Often it will be advantageous to put the GFF firstsince it typically has low insertion loss except near the gain peak(s).Therefore, signals at wavelengths away from the peak will be relativelyunaffected, while those near the peak will likely receive considerablegain from the first stage. This arrangement gives the non-peakwavelengths an extra gain stage before they are subject to considerableattenuation. On the other hand, the gain of the first stage (which istypically very highly inverted) is often high in the blueband (i.e,approximately 1530 nm). However, if the amplifier is operated at anaverage inversion such that the filter is also compensating for a gainpeak in the long wavelength portion of the Erbium gain band, it may beadvantageous to put the attenuator/other. component first if peakattenuation is less than that of the GFF's redband peak. The noiseperformance can be estimated using eqn (1) or well known numericaltechniques. Since noise performance and pump to signal power aretypically trade-offs with respect to the partitioning of gain among thevarious stages, a unique optimum cannot be defined a priori.

The present invention also applies when a very high attenuation GFF(peak attenuation, e.g., >10 dB) filter is needed. In this case, the GFFcan be split into multiple filters whose composite attenuation is equalto the total attenuation spectrum which will provide the desired gainshaping performance. The various filters can then be inserted betweenmultiple stages. The gain flattening filter would probably be splitbetween gain stages so that the constituent filter had less peakattenuation than the aggregate flattening required. Alternatively, thissplit could be for the purpose of simplifying the fabrication of adifficult GFF.

An optical amplifier system 30 of this type is schematically illustratedin FIG. 3. Optical radiation to be amplified enters the amplifier systemat the input port 32. The amplified radiation exits at the output port34. The amplifier system 30 comprises three gain stages whose powergains are designated G₁, G₂ and G₃. Each gain stage may be an erbiumdoped optical waveguide segment which is pumped by a pumping laser.

In the system 30, T₁ represents the power transmission coefficient forall components (if any) between the input port 22 and the first gainstage G₁. There is a first gain flattening subfilter 36 between G₁ andG₂ and a second gain flattening subfilter 38 between the gain stages G₂and G₃.

Some illustrative values for a particular embodiment of the system areas follows:

peak after subfilter 36: 7 dB @ 1530 nm

peak after subfilter 38: 6 dB @ 1558 nm

Each subfilter 36, 38 could be made using thin filter interferencefilter technology (or long period fiber gratings, etc.)

In this case, one should account for the spectral evolution of the gainamong the various gain stages and insert the sub filters to keep thespectrally dependent noise factor as uniform as possible. In typicalamplifiers with highly inverted front ends, this translates intoattempting (to the extent permitted by the filter decompositionsavailable) to attenuate the wavelengths near the Erbium fluorescencepeak first and reserving the gain flattening attenuation of the longerwavelength regions (which build up total gain more slowly) for later inthe amplifier.

Finally, the above described embodiments of the invention are intendedto be illustrative only. Numerous alternative embodiments may bedescribed by those skilled in the art without departing from the scopeof the following claims.

What is claimed is:
 1. An optical amplifier suitable for use in acommunication system, said amplifier comprising: multiple gain stagessituated proximate to one another, said multiple stages arrangedserially for providing gain to an optical signal propagating therein; again flattening filter for reducing a variation in gain of said opticalamplifier in a particular wavelength band; and an optical attenuationelement, wherein (i) one of said gain stages is located between saidgain flattening filter and said optical attenuation element to reduce anoverall noise factor of said optical amplifier and (ii) no gainflattening filters are situated adjacent to any optical attenuationelements.
 2. The optical amplifier of claim 1 wherein each of said gainstages comprises a pumped erbium doped optical waveguide segment.
 3. Theoptical amplifier of claim 2 wherein the attenuation element precedesthe gain flattening filter in a direction of propagation of an opticalsignal in said amplifier.
 4. The optical amplifier of claim 2 whereinthe gain flattening filter precedes the attenuation element in adirection of propagation of an optical signal in said amplifier.
 5. Again flattened optical amplifier suitable for use in a communicationsystem, said amplifier comprising: a plurality of optical gain stagessituated proximate to one another, and a plurality of gain flatteningfilter stages wherein said gain flattening filter stages and said gainstages are arranged serially in an alternating manner and no gainflattening filter stages are situated adjacent to any opticalattenuation elements.
 6. The optical amplifier of claim 5 wherein eachof said gain stages comprises an erbium doped optical waveguide segment.7. An optical fiber amplifier suitable for use in a communicationsystem, said amplifier comprising: an input port through which a firstoptical signal enters said amplifier system; an output port throughwhich a second optical signal exits said amplifier system; at leastfirst, second, and third gain stages arranged serially between the inputport and the output port and situated proximate to one another, each ofsaid gain stages comprising an optical waveguide segment doped with anelement for providing optical gain; a gain flattening filter connectedbetween said first gain stage and said second gain stage; and an elementwith optical attenuation connected between said second gain stage andsaid third gain stage.
 8. The amplifier of claim 7 wherein said firstgain stage is nearest said input port and said third gain stage isnearest said output port.
 9. The amplifier of claim 7 wherein said firstgain stages is nearest said output port and said third gain stage isnearest said input port.
 10. The amplifier of claim 7 wherein each ofsaid first, second and third gain stages comprises a segment of erbiumdoped optical waveguide.
 11. The amplifier of claim 7 wherein theelement with optical attenuation includes a variable optical attenuator.12. The amplifier of claim 7 wherein the element with opticalattenuation includes a switch.
 13. The amplifier of claim 7 wherein theelement with optical attenuation includes an add-drop element.
 14. Theamplifier of claim 7 wherein the element with optical attenuationincludes at least one of an variable optical attenuator, an isolator, anadd multiplexer, and a drop multiplexer.
 15. An optical amplifiercomprising: multiple gain stages situated proximate to one another andarranged serially for providing gain for an optical signal propagatingtherein, each gain stage comprising a pumped segment of opticalwaveguide, a gain flattening filter for reducing a variation in gain ofsaid optical amplifier in a particular wavelength band, and an opticalattenuation element, wherein (i) one of said gain stages is locatedbetween said gain flattening filter and said optical attenuationelement; (ii) no gain flattening filters are situated adjacent to anyoptical attenuation elements, wherein gain values are chosen for saidstages so as to achieve an improvement of one of a noise figure and apump power requirement at a fixed value of the other.
 16. An opticalamplifier comprising: multiple gain stages arranged serially providinggain to an optical signal propagating therein, said signal being in atleast the 1530 nm to 1560 nm wavelength range; at least one gainflattening filter for reducing a variation in gain of said opticalamplifier in a particular wavelength band within said wavelength range;and at least one optical attenuation element, wherein there are noattenuation elements located directly adjacent to said gain flatteningfilter; and wherein one of said gain stages is located between said gainflattening filter and said optical attenuation element to reduce anoverall noise factor of said optical amplifier.
 17. The opticalamplifier of claim 16 wherein each of said Slain stages comprises apumped erbium doped optical waveguide segment.
 18. The optical amplifierof claim 17 wherein the attenuation element precedes the gain flatteningfilter in a direction of propagation of an optical signal in saidamplifier.
 19. The optical amplifier of claim 17 wherein the gainflattening filter precedes the attenuation element in a direction ofpropagation of an optical signal in said amplifier.
 20. A gain flattenedoptical amplifier comprising: a plurality of optical gain stagesamplifying signals in at least the 1530 nm to 1560 nm wavelength range,and a plurality of gain flattening filters, wherein said gain flatteningfilter stages and said gain stages are arranged serially in analternating manner, so that every gain stage has an adjacent gainflattening filter and there are no attenuation elements located adjacentto any of said gain flattening filters.
 21. The optical amplifier ofclaim 20 wherein each of said gain stages comprises an erbium dopedoptical waveguide segment.
 22. An optical fiber amplifier comprising: aninput port through which a first optical signal enters said amplifiersystem; an output port through which a second optical signal exits saidamplifier system; at least first, second, and third gain stages arrangedserially between the input port and the output port, each of said gainstages comprising an optical waveguide segment doped with an element forproviding optical gain for the optical signal in the 1530 nm to 1560 nmrange; a gain flattening filter connected between said first gain stageand said second gain stage; and an element with optical attenuationconnected between said second gain stage and said third gain stage,wherein there is no gain flattening filters located next to an opticalattenuation element.
 23. The amplifier of claim 22 wherein said firstgain stage is nearest said input port and said third gain stage isnearest said output port.
 24. The amplifier of claim 22 wherein saidfirst gain stages is nearest said output port and said third gain stageis nearest said input port.
 25. The amplifier of claim 22 wherein eachof said first, second and third gain stages comprises a segment oferbium doped optical waveguide.
 26. The amplifier of claim 22 whereinthe element with optical attenuation includes a variable opticalattenuator.
 27. The amplifier of claim 22 wherein the element withoptical attenuation includes a switch.
 28. The amplifier of claim 22wherein the element with optical attenuation includes an add-dropelement.
 29. The amplifier of claim 22 wherein the element with opticalattenuation includes at least one of an variable optical attenuator, anisolator, an add multiplexer, and a drop multiplexer.
 30. An opticalamplifier comprising: multiple gain stages arranged serially forproviding gain for an optical signal propagating therein, each gainstage comprising a pumped segment of optical waveguide, wherein saidoptical signals are at least in the 1530 nm to 1560 nm range; a gainflattening filter for reducing a variation in gain of said opticalamplifier in a particular wavelength band, and an optical attenuationelement, wherein one of said gain stages is located between said gainflattening filter and said optical attenuation element and no opticalattenuation elements are located directly adjacent to said gainflattening filters, and wherein gain values are chosen for said stagesso as to achieve an improvement of one of a noise figure and a pumppower requirement at a fixed value of the other.
 31. The opticalamplifier according to claim 16, wherein said multiple gain stagesinclude a first gain stage G₁ that provides high gain of at least 15 dB.32. The optical amplifier according to claim 31, wherein said gain G₁is: 15 dB≦G₁≦20 dB.
 33. The optical amplifier according to claim 31,wherein other gain stages provide gain that is 15 dB or smaller.
 34. Theoptical amplifier according to claim 31, wherein said first gain stageG₁ is high in 1530 nm region.